The present invention relates to fabricating processes for joining metal components and, more particularly, to a technique of joining metal components by utilizing retrogressive heat and magnetic bonding techniques.
It is often desirable to join lightweight aluminum alloy tubes to aluminum or steel fittings for the purpose of transmitting torque or axial forces. It is desirable to employ steel end fittings in some cases, because of the great applied forces. For example, the forward end of a torque transmitting driveshaft is splined. The spline is inserted into the aft end of the transmission. It slides in and out of the transmission during the course of the normal shifting operation, while the suspension of the automobile moves up and down. The combined torque and vertical movement increases stress and strain upon the spline, therefore making steel often the preferred choice of material. Steel is also significantly less expensive than aluminum. Where weight is of primary importance, however, aluminum end fittings which are one third the weight of steel are used.
Aluminum cannot be joined to steel using conventional welding methods. Other methods have proven to be difficult and expensive to attach an aluminum tube to a steel end fitting in most applications that require significant loads and torque. Bonding, friction welding, and mechanical fastening have been employed with various degrees of success, but the joined materials generally have a shortened operational life. Conventional welding can be used to join aluminum tubes to aluminum end fittings but because heat is inherent in the process, distortion and localized annealing or weakening of the aluminum tube occurs. This requires that the tube is thickened to compensate for this effect. This results in a heavier, more expensive assembly.
The present invention reflects the discovery of a method of accomplishing the joining of both similar and dissimilar metals by a new electromagnetic attachment of the tube to the fitting. Electromagnetic forming (EMF) or magnetic pulse welding (MPW) has been used in the past with some success, where the materials are not required to carry high or combined load forces. In these processes, electromagnetic fields are created around the tube which create an opposing field in the tube. The two fields repel one another, causing the tube to deform inwardly onto the end fitting.
With the use of MPW, the tube forms a molecular bond with the end fitting. In the case of EMF, a mechanical joint is formed, which deforms the tube into a groove pattern disposed in the end fitting. The electrical energy required to perform the MPW or EMF processes successfully, in order to join metals, increases as the yield strength of the material increases. Therefore, a problem arises when the metal being deformed is high strength aluminum. In addition, the ductility of the material generally decreases as yield strength increases, making it still more difficult to form a successful EMF or MPW joint using high strength aluminum alloys, such as 6061-T6, 6013-T6, 2024-T3, or metal matrix composite.
In order to join a high strength aluminum tube to an aluminum or steel fitting, the inventive process subjects the aluminum tube to local regressive heat treatment (RHT), prior to electromagnetic deformation. In other words, the aluminum is subjected to inductive heating prior to the electromagnetic forming or magnetic pulse welding. The RHT process involves elevating the temperature of the material to approximately one thousand degrees Fahrenheit and then quenching the aluminum in water. The regressive heat treatment reduces the yield strength of the tube, while increasing its ductility for a period of several hours. The aluminum, which has been subjected to the RHT process gradually regains its temper. Eventually, the aluminum tube develops properties greater than T4 aluminum, but less than those of T6 aluminum. The strain induced in the material by the EMF and MPW processes locally increases the temper of the material, due to work hardening. After the EMF or MPW process, the material may be subjected to additional heat or an aging process in order to increase the temper of the tube to a final, higher temper value.
During the inductive heating process, it has been noted that the hardness readings about the circumference of the tube are not uniform. Consistent hardness in the heated zone is critical to a consistent welded or formed part. Rotating the tube in the induction-heating coil either manually or preferably using a motorized fixture obtains uniform hardness readings.
In U.S. Pat. No. 3,603,759, issued on Sep. 7, 1971 to Peacock, for WELDING AND FORMING METHOD, magnetic forces are utilized to force members together at an interface.
In U.S. Pat. No. 3,944,641, issued to Lemelson on Mar. 16, 1976, for PROCESS OF FORMING AN IRREGULAR SURFACE ON THE INSIDE OF A TUBE OR PIPE, an apparatus and method are illustrated for providing the inside of a pipe with an irregularly shaped surface. An inner liner of the pipe is extruded or subjected to an electromagnetic field, in order to vary the surface configuration or roughness of the internal surface. In U.S. Pat. No. 5,218,763, issued on Jun. 15, 1993 to Marker et al, for METHOD FOR MANUFACTURING A SWAGED PISTON ASSEMBLY FOR AN AUTOMOBILE AIR CONDITIONING COMPRESSOR, a 1018 steel alloy connecting rod is fabricated with an aluminum alloy head. Cold swaging is used to flare the tip of the connecting rod outwardly, in order to conform and fill a counterbored surface. This provides a strong attachment joint between the parts.
In accordance with the present invention, there is provided a method by which similar or dissimilar metal components can be joined together. The metal components carry high loads, or are subjected to combined stresses and strains, thus requiring the use of high strength materials. Electromagnetic forming or magnetic pulse welding when used to join the components, cannot be accomplished easily when the materials have high yield strength and ductility. Therefore, in order to electromagnetically form or magnetic pulse weld these high strength materials, the high strength material is temporarily diminished prior to the joining step. The high strength material is subjected to a retrogressive heating and quenching process, prior to the joining process. This retrogressive heat treatment (RHT) reduces its ductility and yield strength. Thereafter, the material is restored to its higher temper and strength by work hardening and subsequent heat or aging processes.
During the retrogressive heat treatment, while the tube is in the inductive heating coil, the tube may be rotated to improve the consistency of the tube temper around the circumference of the tube in the RHT area.
It is an object of this invention to provide an improved process of joining similar or dissimilar metals using electromagnetic forming or magnetic pulse welding.
It is another object of the invention to provide a process of joining high strength components by moderate electromagnetic forces.